I had a very strange one once.
I was booked in WT, checked online before and found that I was upgraded to WT+
Got to the airport and asked how much to upgrade to CW and was told £185, which I jumped on! Thinking about it later, I guess she must have been working it out from my underlying W fare which would have just shown her that the upgrade price was £185 (which is about what it is usually WT to WT+ on that route) and then stuck me in CW!
Not quite the same as W-->CW for free, but if it happened every time I'd be delighted!